Claisen rearrangement of N-silyl ketene N,O-acetals generated from allyl N-phenylimidates

Article abstract of DOI:10.1016/S0040-4020(01)89670-8

Deprotonation and subsequent N-silylation of allyl N-phenylimidates 1 lead to N-silyl ketene N,O-acetals 5 which undergo a Claisen rearrangement to yield γ,δ-unsaturated anilides 3 after hydrolytic work-up. The temperature for the rearrangement step is dependent on the nature of the substituent R² in 5. Ketene acetals 5 with R²=H rearrange readily at room temperature, while heating at 130°C is required if R²≠H. The degree of simple diastereoselection attainable for the conversion of 1 to 3 is strongly affected by the size of the substituent R⁴. With R⁴>H excellent anti/syn selectivity is caused by efficient suppression of the rearrangement pathway via a boat transition state. This rationale is supported by NOE difference data obtained for several allyl N-phenylimidates and N-silyl ketene N,O-acetals.